The present invention relates generally to generation of electric power and, more particularly, to a method and apparatus for increasing reliability in high altitude starting of aircraft auxiliary power units having gas turbine engines.
Gas turbine engines have typically been employed as auxiliary power units (APUs) in aircraft to provide, for example, a.c. electric power. In conventional APUs, the gas turbine engine has a rotatable drive shaft which is connected to a gear box for transferring motive force to and from the drive shaft. The gear box often includes a variety of speed reduction gears and pads to connect to various accessories. These accessories can include the APU fuel pump, exhaust fan, starter motor, a.c. output generator and gear box lubrication pump. The starter motor can be driven by d.c. current from aircraft batteries or, more recently, from various a.c. current sources.
For safety reasons, it has been important that APUs start quickly and reliably when the aircraft is operating at high altitudes. Due to the size and complexity of the conventional gear box, its lubricating oil would impose a significant vicious drag against high altitude starting. To overcome that drag, a substantial amount of electric power would be required by the starter motor. To avoid that power drain and its potential detrimental effect on other aircraft electric power systems, it has been suggested to employ air/oil separation devices and lubrication oil scavenge pumps for removing lubricating oil from gear box components during APU shut down.
However, adding such components increases both the cost and weight of APUs. Further, in order to avoid viscous drag problems associated with starting the lubrication pump it may be necessary to include a de-priming valve for that pump, thus also increasing APU weight.
In general, prior APUs also evolved toward a line replaceable unit (LRU) configuration where many functional components were separately housed and independently replaceable. The advantage of these configurations was intended to be in repair cost savings since a deficiency in one part could be resolved quickly and without the cost of or replacement time for the entire APU. However, as more and more parts have been added to improve and extend APU performance, the APU weight has significantly increased, due to not just the functional parts, but also their associated housings, bearings, etc. Moreover, it has been found that certain failures of these LRU parts still detrimentally affect adjacent parts such that replacement of the several adjacent parts is typical with replacement of the initially failing part. For example, failure of the starter motor can cause contamination of the gear box, requiring dissociation of all accessories and loads.
Another disadvantage in some prior APUs arises with the incorporation of the APU fuel pump with the gear box. Briefly, to minimize overall weight it has been suggested to drive the fuel pump off of the drive shaft of the gas turbine engine with a speed reduction gear therebetween. Thus, the fuel pump speed is controlled by the drive shaft speed. In order to assure quick starting, the fuel pump has often been sized to provide the optimum fuel supply at low APU speeds. However, at high speeds a lower proportion of fuel is needed. Therefore, some prior APUs either bypassed the excess fuel or burned too rich an air/fuel mixture in the combustion chamber of the gas turbine engine. Bypassing the excess fuel again increases system weight, cost and complexity and also unnecessarily heats the fuel supply.
Accordingly, it is an object of this invention to provide improvements in the generation of electric power. Further objects include, but are not limited to, the provision of a method and apparatus for:
1. more reliable high altitude starting for aircraft APUs, PA1 2. decreased aircraft APU weight, PA1 3. decreased aircraft APU initial and servicing costs, PA1 4. decreased aircraft APU fuel consumption, PA1 5. streamlined construction, operation and maintenance of APUs, PA1 6. greater operational control over APUs, PA1 7. decreased APU spacial requirements, and PA1 8. improved APU operational efficiency.
These and other objects of the present invention are attained by the provision of an integrated APU having a combined starter/generator device mounted within the gearbox housing so as to reduce the number of reduction gears, bearings and other redundant components sufficiently to eliminate the need for a separate lubrication oil scavenger pump as well. Further integration and savings can be achieved by de-coupling the fuel pump and lubrication pump from the gearbox and driving those pumps instead by an electric motor.
Other objects, advantages and novel features will become readily apparent from the attached drawings and detailed description below.